It is desirable, and in certain places mandated by law, that mobile telecommunication network providers be able to determine an approximate geographical location of a mobile terminal (MT), such as, for example, an actively communicating cellular telephone.
There are a variety of MT location techniques currently being tested or used. These location techniques can be grouped into three basic categories.
The first basic category includes "uplink signal" location techniques, wherein the mobile telecommunications network is configured to determine where the MT is located based on ranging measurements associated with one or more uplink signals, which are transmitted by the MT and received by a requisite number of receivers having known locations, such as, for example, cellular telephone base stations (BSs).
The second basic category includes "downlink signal" location techniques, wherein the mobile telecommunications network is configured to determine where the MT is located based on ranging measurements associated with the reception, by the MT, of downlink signals from a requisite number of transmitters having known locations.
The third basic category includes using location services not associated with either the uplink or downlink signals used in the mobile telecommunications network. One example, of such a location service is the Global Positioning System (GPS) in which GPS receivers collect and analyze ranging measurements from signals transmitted by GPS satellites having known locations. Currently, there are twenty-four (24)GPS satellites in orbit.
The location techniques in each of these three basic categories include collecting ranging measurements such as, for example, a time of arrival (TOA), a time difference of arrival (TDOA), an observed time difference (OTD), or the like. These ranging measurements are gathered by detecting one or more measurement features within the transmitted/received signal(s). Each of the various location techniques has certain limitations or drawbacks that can significantly reduce their accuracy.
By way of example, currently available or proposed TOA, TDOA, and OTD location techniques that utilize existing BSs typically require that at least three (3) or more BSs receive the transmitted uplink signal from the MT, or conversely that the MT receive transmitted downlink signals from at least three BSs to perform the locating process. Similarly, with respect to the GPS, a GPS receiver needs to receive transmitted signals from at least four (4) GPS satellites to perform the locating process.
Unfortunately, at certain times there is not always a clear line-of-sight (LOS) between the requisite transmitter(s) and receiver(s). For example, in an urban environment, the LOS is often blocked by building and/or other structures, while in certain other environments the naturally occurring terrain and/or other features (e.g., mountains, canyons, forests, weather, etc.) can reduce the LOS, attenuate the transmitted signals, or produce multipath signals at the receiver. For many higher frequency signals or weaker signals, the loss of LOS or the introduction of such obstacles, can render the location technique significantly inaccurate, or completely unavailable.
Consequently, there is a need for methods and arrangements that provide location techniques having improved accuracy, reliability, and/or accessibility.